Structures having underwater surfaces or surfaces in contact with water, such as ships and liquid transport channels, produce a flow of water along their surfaces as the ships travel or liquid moves, with the velocity of water being zero at the submerged surface of the structure. A velocity gradient near the structure's submerged surface depends on the viscosity of fluid and the shearing forces due to viscosity constitute the fluid frictional drag.
This fluid frictional drag experienced by the ship can be very large. Of the drags exerted on the ship as it navigates, which include a wave-making drag, a form drag and an air drag in addition to the fluid frictional drag, the fluid frictional drag accounts for as much as 60 to 70% of the total drag or resistance.
Considering such a large proportion of the fluid frictional drag, the best way to economize the propelling power and increase the speed of a ship is to minimize the fluid frictional resistance and the development of such a technology has been strongly called for in the industry.
Among the measures currently employed to reduce the fluid frictional drag of the ship are: a method which provides the bottom of the ship with projections to form frame-like air chambers and supplies air to the air chambers to form a layer of air over the surface of the ship's bottom; and what is generally called a micro-bubble method which blows out air over the surface of the hull to produce a large number of air bubbles over the surface.
With the former method, however, there is a problem that when a ship moves, the air contained in the air chamber flows out and replenishing the lost air requires a large power.
The latter method requires a large-scale compressor because friction resistance can only be reduced when a large quantity of air is supplied from nozzles provided at the side and bottom of the hull. This inevitably increases equipment cost and requires high power to drive the large-capacity compressor. While the supplied air that flows in the form of bubbles along the hull side and bottom reduces the fluid friction resistance, it should be noted that as the bubbles flow downstream, they repetitively combine together and merge into larger bubbles, reducing the area of the bubbles covering the hull and therefore the fluid friction resistance reduction effect. Hence, to obtain a significant reduction in the overall frictional resistance of the entire ship requires supplying a large mount of air, making it difficult to achieve a substantial reduction in the fluid friction drag. For these reasons, either of these methods cannot be put to practical use.
Floating structures such as anchored ships and barges or stationary structures such as underwater steel piles have problems that the surfaces of their submerged portions often have underwater organisms deposited thereon or are attacked by corrosion. As a means to solve these problems, it is a known practice to apply anticontamination or anticorrosion coating to the underwater surfaces. This method, however, not only requires troublesome maintenance of coating but poses a risk of noxious substances in the coating dissolving into water, resulting in environmental pollution.